1. Field of the Invention
This invention relates to a semiconductor light emitting device, and more particularly relates to a semiconductor light emitting device which includes semiconductor light emitting layers placed on a conductive substrate.
2. Description of the Related Art
Semiconductor light emitting devices like semiconductor light emitting diodes are much in demand at present. Especially, semiconductor light emitting diodes where nitride semiconductors are used as light emitting layers can emit light beams of which wave lengths range from those of ultraviolet to green light beams.
Recently, a technology for forming light emitting layers, made of nitride semiconductors, on conductive substrates of semiconductor light emitting diodes is under development. The technology is effective in increasing flexibility in device designing. Further, conductive substrates are relatively inexpensive. Still, further, the technology is preferable since semiconductor light emitting devices can be fabricated using an existing silicon semiconductor manufacturing process. Silicon (Si) substrates, silicon carbide (SiC) substrates and so on are usable as conductive substrates.
However, it is difficult to make the nitride semiconductor on the conductive substrate and to obtain excellent crystalline light emitting layers. For instance, a layer having high electric resistance is created near an interface between a surface of the conductive substrate and the light emitting layer or near an infant light emitting layer. In such a case, the semiconductor light emitting diode tends to increase its forward voltage, decrease its light emitting efficiency, become heated, and have a short life.
Japanese Patent Publication No. 2,741,705 discloses a semiconductor light emitting diode. In the publication, a light emitting layer is placed on a silicon carbide substrate via a layer having high electric resistance, the light emitting layer and silicon carbide substrate are electrically connected by a metal short circuiting electrode, and the layer having the high electric resistance is bypassed using the metal short circuiting electrode. The semiconductor light emitting diode can suppress an increase of the forward voltage.
However, the cited semiconductor light emitting diode seems to have the following problems. The metal short-circuiting electrode is present on a side surface of the semiconductor light emitting layer. Since the semiconductor light emitting layer emits light via its upper and side surfaces, the light emitted via the side surface is blocked by the metal short circuiting electrode. This means that light emitted via the side surface of the semiconductor light emitting diode cannot be effectively utilized, and that the semiconductor light emitting diode seems to suffer from reduced light emitting efficiency.
The present invention has been contemplated in order to overcome the foregoing problems, and provides a semiconductor light emitting device which can reduce a forward voltage between a conductive substrate and semiconductor light emitting layers, and improve the light emitting efficiency.